Carbon-carbon fiber composite materials

ABSTRACT

Defective, e.g. worn or substandard, carbon-carbon fibre composite materials such as frictional materials in brakes or clutches may be restored and/or upgraded by impregnation with a molten reactive metal source comprising a carbide-forming metal or a derivative thereof such as an oxide.

This invention is concerned with a method for enhancing the propertiesof carbon-carbon fibre composite materials and structures, moreparticularly with a method for the restoration and/or upgrading ofdefective carbon-carbon fibre composites.

Carbon-carbon fibre composite materials and structures, e.g. comprisinga matrix of carbon embedded in a carbon fibre structure, are used interalia as frictional materials in brakes and clutches. In suchapplications these materials may experience frictional and oxidativewear, leading to loss of both physical dimensions and physicalproperties such as strength and thermal capacity. As a result ofdegradation of their physical properties such materials may have to bediscarded as defective when only partly worn in terms of their physicaldimensions.

Defective or substandard carbon-carbon fibre composites may also resultfrom faulty manufacturing processes or as a result of stresses imposedwhen used in applications other than as frictional materials. Thus, forexample, faulty manufacturing processes often lead to low densityproducts with fine crazing and shrinkage cracks; such materialstypically exhibit poor mechanical properties and low thermalconductivity and diffusivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: shows a schematic representation of a carbon-carbon fiber brakesample.

FIG. 2: shows a schematic representation of the container system of theinstant invention.

The present invention is based on the unexpected finding that defectivecarbon-carbon fibre composites may be restored/upgraded by treatmentwith a molten reactive metal source. Whilst we do not wish to be boundby theoretical considerations, it would appear that such treatmentresults in intercalation or infiltration of molten reactive metal intopores and interstices within the carbon matrix material and reactionbetween the metal and the matrix carbon to form metal carbide whichreinforces and enhances the properties of the matrix structure, forexample restoring or improving physical and frictional properties suchas structural strength and specific heat capacity, and/or enhancingchemical properties such as oxidation resistance. The carbon fibrecontent of the composite, on the other hand, is surprisingly resistantto reaction with the molten reactive metal and remains substantiallyunchanged throughout the process, so that its own reinforcing functionis not compromised.

Thus according to one aspect of the invention there is provided aprocess for enhancing the properties of defective carbon-carbon fibrecomposite material which comprises impregnating said material with amolten reactive metal source.

Defective composite materials which may be treated in accordance withthe invention include used materials and unused but substandardmaterials, particularly frictional materials. The process may beemployed to restore or upgrade such materials so that they exhibitimproved or different properties and may be used in their originallyintended applications or in new applications.

In general, the process will lead to formation of metalcarbide-containing products, for example comprising a stoichiometricmetal carbide or a mix of free metal, metal carbide and residual carbonmatrix in appropriate proportions, in each case retaining the originalcarbon fibre reinforcing structure.

Impregnation of the composite material with molten metal may, forexample, be achieved using either surface tension or external pressureto drive the metal into pores and interstices within the carbon matrix.Thus impregnation may be effected either by at least partially immersingthe material in a bath of the molten reactive metal source (the moltenbath process), or by encapsulating the material together with excessreactive metal source in an evacuated container, and subjecting thiscontainer to high temperature and isostatic pressure (the encapsulationprocess).

The molten bath process is preferably carried out in an inertatmosphere, e.g. under an inert gas such as argon. The bath may beheated by conventional means such as gas or resistive, microwave orinductive electrical heating. The bath may be supported by or be part ofan oven of appropriate size. If desired, the sample and reactive metalmay independently be heated up to temperatures at or above the melttemperature of the reactive metal source.

In the encapsulation process, on the other hand, the carbon-carbon fibrecomposite sample is encapsulated together with solid reactive metalsource in a sealed container, which is thereafter placed in a hotisostatic press. This specifically designed equipment enables hightemperatures and high isostatic pressures to be applied, over controlledtime periods, to the container and its contents. An inert gas is usuallychosen as the pressurising fluid. The material from which the containeris formed is chosen so that it may become ductile at the temperature andpressure required for reaction to proceed in the container; theresulting softening allows the pressure applied to the outside of thecontainer to be transmitted to the contents thereof, effectively forcingmolten metal into interstices and pores within the carbon matrixstructure.

Reactive metals useful in the process of the invention are metals whichmay form one or more carbides and which may be contained in a meltvessel or encapsulated by a material of higher melting point. Suchmetals include those which are themselves refractory, for example metalswhose melting point is 1,300° C. or higher. The metal may, for example,be a member of Group III, IV, V or VI of the Mendeléef Periodic Table ofElements, representative examples being shown in the following Table I:

TABLE I Group of the Melting Point Mendeléef Periodic Metal Symbol (°C.) Table of Elements Silicon Si 1,420 IVb Titanium Ti 1,850 IVa HafniumHf 2,500 IVa Molybdenum Mo 2,500 VIa Tantalum Ta 2,900 Va Tungsten W2,900-3,000 VIa Zirconium Zr 2,130 IVa Boron B 2,300 IIIb

It will be seen that the melting points of these metals fall within therange 1,400° C. to 3,000° C.; most are in the range 1,800° C. to 3,000°C.

As an example for purposes of this invention silicon may be used as theimpregnating and reacting metal and the samples may be encapsulated byzirconium or molybdenum. Carbon, which does not melt but sublimes atatmospheric pressure and temperatures in excess of 3,500° C., may alsobe used as an encapsulating material.

The source of reactive metal may, for example, be the metal itself or asuitable derivative, e.g. an oxide, thereof. The metal source inencapsulation processes may conveniently be in thin sheet, powder orother finely divided form.

If desired, the reactive metal source may comprise two or morecomponents, for example two or more metals, or may contain one or moreadditives capable of dissolving in the molten metal or metal alloys.

Carbon-carbon fibre composites for treatment in accordance with theinvention may typically have a porosity of at least 10%; in the case ofarticles which have undergone substantial frictional and/or oxidativestress this may often be in excess of 15%. Whilst it may be advantageousif the pores and interstices form connected voids, it is in practiceonly necessary for the composite material to have a degree of surfaceporosity, since infiltrating molten metal will be capable of forming areaction front which progressively penetrates into the carbon matrix,driven by a combination of chemical reaction forces and physical forcessuch as surface tension and/or applied pressure.

The following conditions are important to operation of the process:

a) the melt temperature of the reactive metal source;

b) the porosity of the composite material to be treated; and

c) the infiltration and reaction times of the reactive metal with thecarbon matrix.

For a given matrix material and reactive metal, the infiltration andreaction rates may be varied by controlling the temperature and, whereappropriate, pressure applied over a controlled timescale. In the caseof a molten bath process, reaction rate and extent of infiltration areprimarily controlled by temperature and time. In the case of anencapsulation process, pressure, temperature and time are used ascontrolling parameters.

A typical hot isostatic pressing procedure comprises placing theencapsulated sample in a pressure vessel adapted to receive a gas athigh pressure and also adapted to be heated to a suitable temperature.After the encapsulated sample has been inserted into the vessel, thelatter is sealed and the gas pressure and temperature increased atpredetermined rates until they reach the operating pressure andoperating temperature, at which they are maintained for an operatingtime sufficient to provide a satisfactory product; it will beappreciated that the operating time is only a portion of the total timeduring which the encapsulated sample is subjected to superatmosphericpressure. In general terms, the operating pressure is normally at least500 bar, for example in the range 750 to 2,500 bar, preferably 1,000 to2,000 bar, and the operating temperature is normally at least 1000° C.,for example in the range 1,000° C. to 3,000° C.

Examples of conditions of pressure and temperature which may be used areas follows:

TABLE II Metal Encapsulating Pressure Temperature source material (bar)(° C.) Silicon Zirconium 1,000-2,500 1,400-1,850 Silicon Molybdenum1,000-2,500 1,400-2,000 Silicon Tantalum 1,000-2,500 1,400-2,000Titanium Molybdenum 1,000-2,500 1,680-2,000 Titanium Tantalum1,000-2,500 1,680-2,300 Zirconium Tantalum 1,000-2,500 1,850-2,300 BoronTantalum 1,000-2,500 2,180-2,400 Molybdenum Tungsten 1,000-2,5002,600-3,000 Tantalum Carbon 1,000-2,500 2,950-3,300

A particular advantage of the process of the invention is that, ingeneral, the composite material samples do not suffer from undue changeof shape during the process, any change in dimensions which does occurgenerally being isotropic. This feature is known as retention of“near-net shape”, and maybe assisted by the fact that the carbon fibrecontent of the samples remains substantially unchanged throughout theprocess.

Prior to processing in accordance with the invention, samples such asworn or defective structures may be machined to a desired new shape. Thesamples may additionaly or alternatively be washed, the use ofultrasonic washing to expose their pore structures being advantageous.If desired, new carbonaceous material may be applied to supplement thecarbon matrix prior to the molten metal infiltration process. This maybe achieved using carbonisable organic materials such as suitablepitches or thermosettable polymers, for example phenolic or furfurylresins, e.g. in solution or melt form, or crosslinkable monomers oroligomers, which will be carbonised at temperatures well below themelting point of the molten metal, thus providing extra carbon at themetal infiltration stage.

The invention is illustrated by the following examples in which alltemperatures are in degrees Celsius. It will be appreciated that allprocessing conditions relate to the size and shape of the particularsample, and that larger samples will generally take longer to heat andcool than smaller samples.

EXAMPLE 1

This Example describes the restoration of a used carbon-carbon fibrecomposite brake by reactively impregnating it with molten silicon usinga molten bath process.

A used brake disk was washed and trimmed on a milling machine to removedebris and other surface imperfections and to provide a new frictionsurface at the required new dimensions. The disk was then ultrasonicallywashed to expose the open pore structure, dried in a vacuum oven andweighed. The cleaned brake material was then suspended in a hightemperature oven over a silicon bath, the oven and bath being blanketedwith argon. When the temperature of the disc reached the meltingtemperature of the silicon it was lowered into the molten silicon bathand held so that it was partly immersed. The weight gain of the disc wasthen monitored during the impregnation. The rate of impregnation wascontrolled by the degree of immersion and the temperatures of the moltensilicon bath and sample. The bath temperature was varied between 1450°and 1700°.

When the desired weight gain had been achieved the disc was removed fromthe bath and allowed to cool slowly in an inert atmosphere. If desiredthe disc may then be further impregnated after cleaning up the surfaceto remove excess silicon.

Using the process described, a used carbon-carbon fibre brake disc whichoriginally had a carbon matrix:carbon fibre content of 70% and 30% byvolume respectively, a density of 1.8 kg/m³ and a calculated porosity of10% by volume was converted to a material with an average density of2.45 kg/m³ and a composition of 30% carbon fibre, 46.6% carbon matrix,11.7% silicon and 11.7% silicon carbide. A weight gain of approximately20% was recorded.

EXAMPLE 2

This Example describes the recovery of used brake materials by reactiveimpregnation with molten silicon in an encapsulation process, using adeformable container membrane isostatically to pressurise molten siliconinto the pores of the carbon-carbon fibre composite.

The brake sample was prepared as described in Example 1. Schematicrepresentations of such a brake sample and the container system areshown in FIGS. 1 and 2 of the accompanying drawings.

The brake sample (1) to be treated had a large central hole and a numberof radially drilled holes, the latter for cooling purposes. The centralhole and the radially drilled holes were fitted with loose fitting plugs(2) to reduce the total amount of non-essential silicon required in theprocedure. The plugs were slightly tapered and were made of anon-reactive material such as silicon carbide or non-porous carbonmonolith of low reactivity (see FIG. 1).

The brake sample was then placed in a cold drawn lidable can made ofmolybdenum (3) (see FIG. 2) on top of a consolidated bed of siliconpowder (4) which rested on a well-fitting silicon carbide plate (5) inthe bottom of the can. The amount of silicon metal was selected to bemore than sufficient when melted to impregnate the sample and fill allthe interstices between the brake material and the loose fitting plugs.The brake sample was held flush against a top silicon carbide plate (6)by the lid (7) of the can. The can was then sealed under vacuum, usingelectron beam welding and taking due care to exclude air from the can.

The canned brake sample was then placed in a hot isostatic press whereit was subjected to a range of controlled temperature and isostaticpressure conditions. The temperature of the sample and the essentialisostatic pressure on the sample were raised to 1450° and 1000 bar over2 hours.

During this time the silicon melted and the low viscosity, high surfacetension molten metal started to penetrate the interstices and pores ofthe brake sample, at the same time reacting with the available matrixcarbon. As a result of a 9.5%. reduction in volume of the silicon onmelting and the filling of interstices between the plugs and the brakesample, the overall volume of the components inside the can was reduced.Since molybdenum is soft and ductile at the operating temperature, thecan collapsed around its contents as a sealed flexible envelope, thusallowing the external isostatic pressure to be exerted on the moltensilicon.

In order to enhance this effect and the reaction rate of molten siliconwith the matrix carbon, the temperature and external isostatic pressureon the sample were then slowly raised over 4 hours to 1650° and 2000bar, during which time the molybdenum can further deformed around thesample and pressurised the liquid silicon further into the pores so thatit could impregnate and react with previously unavailable matrix carbon.

The sample was then allowed to cool slowly to 1200° over 4 hours at 2000bar, whereafter both temperature and pressure were reduced over 3 to 4hours to room temperature and pressure.

The treated brake sample was then removed and decanned. The originalloose fitting plugs were then pressed or machined out and the treatedbrake sample was dressed and trimmed to remove artifacts and excesssilicon and to prepare a good friction surface.

Using the process described, a used carbon-carbon fibre brake disc whichoriginally had a carbon-carbon matrix:carbon fibre content of 70% and30% by volume respectively, a density of 1.8 kg/m³ and a calculatedporosity of 10% by volume was converted to a material with an averagedensity of 2.40 kg/m³ and a composition of 30% carbon fibre, 23.3%carbon matrix, 15.6% silicon and 31.1% silicon carbide. A weight gain ofapproximately 33% was recorded.

What is claimed is:
 1. A process for enhancing the properties of usedcarbon-carbon fibre composite material which comprises impregnating saidmaterial with a molten carbide-forming metal or oxide thereof.
 2. Aprocess as claimed in claim 1 wherein the carbide-forming metal is amember of group III, IV, V or VI of the Mendeleef Periodic Table ofElements.
 3. A process as claimed in claim 2 wherein the carbide-formingmetal is silicon.
 4. A process as claimed in claim 1 whereinimpregnation is effected by at least partially immersing thecarbon-carbon fibre composite material in a bath containing the moltencarbide-forming metal or oxide thereof.
 5. A process as claimed in claim1 wherein impregnation is effected by subjecting the carbon-carbon fibrecomposite material and the carbide-forming metal or oxide thereof to hotisostatic pressing at a temperature exceeding the melting temperature ofthe carbide-forming metal or oxide.
 6. A process as claimed in claim 1wherein the carbon-carbon fibre composite material comprises wornfrictional material.
 7. A process as claimed in claim 1 wherein thecarbon-carbon fibre composite material is machined to a desired shapeprior to impregnation.
 8. A process as claimed in claim 1 wherein thecarbon-carbon fibre composite material is subjected to ultrasonicwashing prior to impregnation.
 9. A process as claimed in claim 1wherein carbonisable organic material is applied to the carbon-carbonfibre composite material prior to impregnation.